Synchronous electric motor with magnetised rotor and method of manufacturing this motor

ABSTRACT

A synchronous electric motor includes least one flat annular rotor part, magnetized so as to present at least one series of magnetic poles arranged along a flat annular zone of the rotor part. The rotor part is fixed by at least one rotor-supporting part on a shaft of the motor. The motor shaft is mounted for rotation with respect to at least one stator assembly which includes at least two stator parts. The stator parts include pole parts arranged so as to form, in their assembly, an air-gap in which are disposed the magnetic poles of the rotor part. At least one electric control coil being coupled with one or more of the pole parts.

The present invention relates to a synchronous electric motor comprisinga least one rotor part of essentially flat annular form, magnetised soas to present at least one series of magnetic poles disposed regularlyalong a planar annular zone, the said rotor part being fixed by means ofat least one rotor-supporting part on a motor shaft mounted to berotatable relatively to at least one stator assembly, the statorassembly comprising at least two stator parts of a magneticallypermeable material, these forming, being fast with or being coupled withpole parts arranged so as to form, in their assembly, a variable air gapalong at least one portion of an annular space in which are disposed themagnetic poles of the rotor part, the stator assembly comprising, inaddition, at least one electric control coil coupled with one or severalof the said pole parts. Such a motor is described, for example, inInternational Patent Application No. WO 85/02503.

The invention has for its object to provide a motor of theaforementioned type, of which the conception, while using usual elementssuch as standard ball bearings for the mounting of the motor in relationto the stator assembly, facilitates the fabrication of this motor,particularly at the time of assembly, and makes it possible to achievein a simple and economic fashion a very good precision in thepositioning of the rotor part with respect to the stator assembly.

To this end, the motor according to the invention is characterised inthat the fixing of the rotor part on the rotor-supporting part iseffected in such manner as to permit, when assembling the motor, apositioning of the rotor part in the axial direction and also an angularpositioning of this latter relatively to the direction of the motoraxis.

Preferably, the stator assembly comprises separate stator parts whichform part of a same magnetic circuit, the stator assembly being arrangedso as to permit, at the time of assembling the motor, a positioning ofthe stator parts relatively to one another in the axial direction. Inthis case, the stator parts which form part of a same magnetic circuitmay be interconnected, directly of by means of a yoke part, so as toform a magnetic joint constituted by contact surfaces parallel to theaxis of the motor shaft. More particularly, the stator assembly may beformed by two coaxial annular stator parts of like form, each having atleast one first and one second cylindrical wall portion, the internaldiameter of the first wall portion being equal to the external diameterof the second, the two stator parts being joined in such manner as to befitted one within the other by the said respective first and second wallportions.

The invention is more particularly applicable to a motor in which therotor part is present in the form of an annular disc of small thicknesswith respect to its external diameter and is magnetised axially, and thestator assembly comprises pole parts disposed on both sides of the rotorpart.

Preferably, the stator assembly and/or the rotor supporting partcomprise at least one access opening communicating with the air spacebetween the rotor part and the pole parts, this opening being accessiblefrom outside the stator unit assembled with the motor shaft providedwith the rotor part.

The invention also has for its object a method of fabricating such amotor, in accordance with which, first of all, the motor shaft providedwith the rotor-supporting part is assembled with an assembly of poleparts which have to be disposed facing a corresponding flat annular zoneof the respective rotor part, in such manner that the ends of these poleparts, in the axial direction, are disposed in a plane perpendicular tothe axis of the motor shaft, that the annular rotor part is placed inposition by disposing at least one spacer element of a determinedthickness between the latter and the ends of at least one part of thesaid pole parts, that the rotor part is fixed on the rotor support andthat the spacer part is withdrawn or removed from the space between therotor part and the said pole parts. More particularly, the spacer partmay be a sheet of thermo-retractable plastic material, a sheet of amaterial capable of being dissolved in a chemical solvent or a metallicwire of essentially circular form.

Other features of the invention will become apparent from the claimsformulated and from the following description of different embodimentsgiven by way of example and illustrated in the accompanying drawing, inwhich:

FIG. 1 is an axial section of a first embodiment of the motor accordingto the invention,

FIG. 2 is an axial section of an embodiment which is similar to that ofFIG. 1,

FIG. 3 is a partial sectional view along the line III--III of FIG. 2,

FIG. 4 is an axial section of another embodiment of the motor, and

FIG. 5 is a view, partly in section, along the line V--V of FIG. 4.

FIG. 1 illustrates a first embodiment of a two-phase motor, comprisingtwo stages, each stage representing one of the phases of the motor.

A shaft 1 of the motor is mounted to be rotatable, by means of the twoball bearings 2, 3, in a stator support 4. In order to avoid any axialplay of the shaft, the axial play of the ball bearings is taken up andthe inner race if these latter is welded onto the shaft or the shaft isfixed by means of a circlip 5 and shims (not shown), disposed betweenthe shaft and the adjoining race of the bearing 2. A shoulder 1' of theshaft 1 serves as support for the inside race of the bearing 3.

A rotor-supporting part 6 of plastic material is made fast with theshaft 1, for example, by casting or moulding, as shown in FIG. 1. Thisrotor-supporting part has a certain number of ribs 6', for example,twelve in number, of which the peripheral parts are engraved in acylindrical surface which is coaxial with respect to the shaft 1. Tworotor parts 7, 8, in the form of a thin annular disc, are fixed bybonding on the peripheral parts of the ribs 6', the positioning of theserotor parts being described in greater detail hereafter. The two annulardiscs 7 and 8 are made, in a manner which is well known, of amagnetisable material such as samarium-cobalt, and they are magnetisedaxially so as to present magnetic poles on each of their flat surfaces,for example, poles of which the polarity alternates peripherally of thedisc.

The stator support 4 has an annular reference surface 4', which is flatand situated in a plane pependicular to the axis of the shaft 1 mountedin the stator support. A first stator assembly 9 comprises a lowerstator part 10 and an upper stator part 11 of a magnetically permeablematerial, these two stator parts being of annular form and having acylindrical contact surface 12 parallel to the axis of the motor shaft,so as to form a magnetic joint between the parts 10 and 11. The insideparts 13, 14 of the stator parts constitute assemblies of axially spacedpolar parts and thus present a toothed profile in the direction of thecircumference, so as to form an air-gap of variable height between theparts 13 and 14.

An electric control coil 15 of annular form is disposed between thestator parts 10 and 11 in such manner as to be coupled with thecorresponding pole parts. A second stator assembly 16 is arranged inquite a similar manner to the stator assembly 9 and comprises anelectric control coil 17. The rotor parts 7 and 8 are respectivelydisposed in the air-gaps formed by the stator assemblies 16 and 9.

It is also to be seen in FIG. 1 that the stator assembly 16 is restingon the stator assembly 9, a planar annular contact surface beingprovided for defining the position and particularly the orientationperpendicularly of the axis of the shaft 1 of the stator assembly 16.The stator support 4 comprises a lateral wall, of which the interiorcylindrical surface permits the centering of the stator assembliesrelatively to the motor shaft. Finally, a cover resting on the statorassembly 16 and closing the housing formed by the stator support 4 canbe seen in FIG. 1.

When assembling the motor, the shaft 1 provided with therotor-supporting part 6 is first of all mounted in the stator support 4in the manner already referred to above. The lower stator part 10 of thestator assembly 9 is then positioned on the reference surface 4' and asheet of a heat-retractable plastic material 18 is then placed on theridges of the toothing of the part 14, which sheet may be of annularform and have a determined thickness, for example, of seven hundredthsof a millimeter. Such a heat-retractable material is, for example,ordinary PVC.

The coil 15 is then positioned on the lower part 10 of the statorassembly and the sheet 18 placed in the bottom of this latter. The lowerrotor part 8 is then placed on the ridges of the toothing of the part14, on which ridges is positioned the sheet 18. The rotor part 8 may, tothis effect, be displaced freely along the support part 6, i.e. alongthe peripheral parts of the ribs 6', relatively to which it is centered.The separation sheet 18 thus determines, by its thickness, the axialdistance between the rotor part 8 and the part 14 and, in addition, itassures that the rotor part is disposed parallel to the referencesurface 4' and thus perpendicular to the axis of the motor shaft.

The sheet 18 is then folded around the coil 15 in such manner that theedge or margin of the sheet 18 is placed on the upper surface of therotor part 8. The upper part 11 of the stator assembly 9 is thenpositioned on the sheet 18 covering the coil 15 and the rotor part 8.The sheet 18 thus determines the axial spacing between the rotor part 8and the part 13 of the stator assembly. In this position, the rotor part8 is stuck or fixed by another means on the rotor support part 6 and thetwo stator parts 10 and 11 are also fixed, for example, by adhesion orwelding, relatively to one another and relatively to the stator support4.

The upper stator assembly 16 and the corresponding rotor part 7 areassembled and fixed in a manner simmilar to that which has just beendescribed, the upper surface of the stator part 11 serving as referencesurface for the lower part of the stator assembly 16.

When the two stator assemblies and the two rotor parts are fixed intheir respective positions, an electric current is caused to passthrough the coils 15 and 17 in such a manner as to produce a heating ofthe plastic sheets surrounding these coils and to cause the retractionof these sheets from the air-gaps of the two stator assemblies.

Instead of using a heat-retractable plastic material, it is alsopossible to employ a plastic material capable of being dissolved in asuitable chemical solvent, which is introduced into the air-gaps forobtaining the elimination of the plastic material at this position, sothat the rotor parts are able to turn freely in the air-gaps.

In order to achieve an adjustment between the two phases of a two-phasemotor, the fixing of the upper stator assembly relatively to the statorsupport is effected once the phase relation relatively to the lowerstator assembly has been determined, for example, in the followingmanner. The rotor is caused to turn and the voltages induced in thecoils 15 and 17 are applied to a phasemeter. The angular position of theupper stator assembly 16 is adjusted in order to bring the inducedvoltages into quadrature. The assembly 16 is then definitively stuck inthe stator support 4.

FIG. 2 illustrates another embodiment, which differs from the foregoingin the following respects. The two stator assemblies 209 and 216 andalso the side wall of the stator support 204 have radial openings 220,221 and 222, respectively. These openings can be seen in the top planview which is shown in FIG. 3 and which represents a section along theline III--III of FIG. 2.

In this embodiment, the annular spacing sheet 18 of FIG. 1 is replacedby sheet portions, in ribbon form, which pass through the openings 220,221, 222 and can be withdrawn to the exterior when the stator assembliesand the rotor parts have been fixed. These ribbons are, for example,made of a material named "MYLAR" (Trade Mark), so as to form a spacingelement of a well established thickness, which can easily be completelyremoved, because of the high mechanical resistance of this material. Inthis case, the annular rotor parts rest, for example, on four points ofreference corresponding to the respective openings in the assembles andthe stator support, as shown in FIG. 3.

The rotor support part 206 according to FIG. 2 has a cylindricalperipheral wall 206' and an upwardly open annular hollow 223. At theheight of the annular rotor parts, the wall 206' has radial openings224, 225 of a section slightly larger than the thickness of the rotorparts. For fixing the rotor parts on the supporting part, the hollow 223is filled with a suitable adhesive which, on being heated at the momentof fixing, passes through the openings 224, 225 distributed along theperiphery of the supporting part and effects an easily controllableadhesion.

FIGS. 4 and 5 illustrate another constructional form of the presentmotor, which has a structure similar to that of FIG. 1. In the assemblyaccording to FIG. 4, a first supporting part 406 is similar to the part6 in FIG. 1 and presents a series of ribs 406' similar to the ribs 6'.Rotor parts 407 and 408 are stuck on respective second supporting parts426, 427, which are presented in the form of annular discs, of which theinternal diameter corresponds to the external diameter of thecylindrical envelope or shell of the ribs 406'. The rotor parts arefixed beforehand on these supporting parts 426, 427, which willthemselves be fixed at the time of the assembly of the motor on thefirst support part 406. Each supporting part 426, 427 has a radialopening, such as 428, at a location situated between two consecutiveribs 406'. The radial openings 428 communicate with the correspondingair gaps in which the respective rotor parts 407, 408 are located, andaccessible from the outside of the stator assembly through the spacebetween the neighboring ribs 406'.

The example of FIG. 4 shows the use, as spacing element corresponding tothe sheet 18 or the MYLAR ribbon 218, of a preformed metallic wire, suchas 418, having an essentially circular configuration and being disposedbetween the ridges of the polar parts of each of the stator assembliesand the respective rotor parts. The preferred form of this spacer wirecan be seen in FIG. 5. This wire has a loop 429 and is open at aposition diametrically opposite to the latter. The loop 429 ispositioned so as to pass through the radial opening of the correspondingsupport part, such as the opening 428 of the part 426, this enabling thewire to be withdrawn by exerting a traction on the loop through thecorresponding space between the adjacent ribs of the supporting part406. The wire 418 is preferably slightly magnetisable, so that, at thetime of assembly, it can be positioned and is held in place on the rotorpart, this further facilitating the assembly procedure. The diameter ofthe circle arcs formed by the wire is about equal to the mean diameterof the pole parts of the stator assemblies, such as 416.

The assembly procedure which has just been described in theaforementioned examples and which makes use of the spacing elementsbetween the pole parts and the rotor parts has, in addition to theadvantage from the point of view of the facility of the assembly and ofthe precision of the air spaces between the parts which cooperatemagnetically, the advantage that the total air-gap in which is situateda rotor part of given thickness, is a function of this thickness, whichis able to vary slightly in a series production. Now the increase insize of the air-gap for a larger magnet thickness compensates to a largedegree for the variation in torque per ampere-turn which would beproduced with a constant total air-gap, such as that which is obtainedby the conventional assembly procedures.

We claim:
 1. A synchronous electric motor comprising at least oneessentially flat annular rotor part, magnetized so as to present atleast one series of magnetic poles arranged regularly along a flatannular zone of said rotor part, said rotor part being fixed by means ofleast one rotor-supporting part on a shaft of the motor, said motorshaft being mounted for rotation with respect to at least on statorassembly, said stator assembly comprising at least two stator parts of amagnetically permeable material, said stator parts including pole partsarranged so as to form, in their assembly, an air-gap in which aredisposed the magnetic poles of said rotor part, at least one electriccontrol coil being coupled with one or more of said pole parts, at leastone of said rotor supporting part and said stator assembly having atleast one access opening communicating with said air-gap and beingaccessible from outside of the stator assembly.
 2. A motor according toclaim 1, wherein the stator assembly comprises separate stator partsforming a magnetic circuit and being coupled to one another alongmagnetic contact surfaces extending parallely to the direction of themotor shaft.
 3. A motor according to claim 1, wherein saidrotor-supporting part comprises a plurality of peripheral portions of asurface of revolution coaxial with respect to the motor shaft, saidrotor part being fixed to at least one of said peripheral portions.
 4. Amotor according to claim 1, wherein said rotor-supporting partcomprises, in its peripheral part, a plurality of radial openingdisposed at the height of the rotor part, so as to permit the passage ofan adhesive through these openings for fixing said rotor part on saidrotor-supporting part.